1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus. More specifically, the present invention relates to a sheet processing apparatus and an image forming apparatus that include a punching unit configured to perform punching processing on a sheet.
2. Description of the Related Art
A conventional image forming apparatus, such as a copying machine, a laser beam printer, a facsimile machine, or a multifunction peripheral, may be equipped with a sheet processing apparatus that can perform binding processing for binding sheets on which images are formed (printed) and then perform punching processing for opening punch holes in the sheets being bound.
FIG. 15 illustrates a configuration of a conventional sheet processing apparatus 2400 that performs the above-described punching processing. The sheet processing apparatus 2400 includes a punch unit 2320 configured to perform punching processing, a lateral registration detection unit 2330 configured to detect the position of an edge portion of a sheet in the width direction perpendicular to a sheet conveyance direction, and a shift unit 2340 configured to move the sheet in the width direction. The punch unit 2320 illustrated in FIG. 15 includes a punch guide, a die, and a conveyance guide.
When the sheet processing apparatus 2400 performs punching processing on a sheet, the lateral registration detection unit 2330 detects an edge portion of the sheet in the width direction while the sheet is conveyed. Then, the shift unit 2340 moves the sheet to a position corresponding to a predetermined punching position based on edge portion information obtained by the lateral registration detection unit 2330.
Next, the shift unit 2340 stops the sheet when the position of the sheet coincides with the predetermined punching position. Then, the sheet processing apparatus 2400 causes the sheet to switch back. An upstream edge of the sheet in the sheet conveyance direction abuts a rear edge stopper 2221. In other words, the sheet processing apparatus 2400 corrects a skew of the sheet. Finally, the sheet processing apparatus 2400 performs punching processing on the skew-corrected sheet.
A conventional sheet processing apparatus discussed in Japanese Patent Application Laid-Open No. 2006-347678 is equipped with a punch unit that can form two-hole type punch holes or three-hole type punch holes at predetermined intervals, respectively, according to a sheet size. When a user sets a sheet size via an operation unit, the above-described sheet processing apparatus selects an operational state of the punch unit according to the input sheet size between a state where the punch unit can form the two-hole type punch holes and a state where the punch unit can form the three-hole type punch holes.
FIG. 16 illustrates a die 2200 that can be used for the punch unit configured to selectively form the two-hole type punch holes and the three-hole type punch holes according to a switching of the above-described operational state. The two-hole type punch holes and the three-hole type punch holes, which can be formed using the die 2200, are mutually different in interval between the holes to be formed and total number of the holes to be formed. The die 2200 includes a pair of die hole portions 2218b and 2218d dedicated to formation of the two-hole type punch holes. The die 2200 further includes three die hole portions 2218a, 2218c, and 2218e dedicated to formation of the three-hole type punch holes.
However, in the above-described conventional sheet processing apparatus or in an image forming apparatus associated with the above-described conventional sheet processing apparatus, the sheet processing apparatus opens the two-hole type punch holes in a sheet if the sheet is a small-size sheet (e.g., LTR_R or LGL).
When the sheet processing apparatus opens the two-hole type punch holes in the above-described small-size sheet, as illustrated in FIG. 16, the sheet processing apparatus once stops a conveyed sheet P at a sheet stop position. Then, the sheet processing apparatus corrects a side edge position of the sheet P in the width direction perpendicular to the sheet conveyance direction. Then, the sheet processing apparatus causes the sheet P to switch back and abut the rear edge stopper 2221.
However, when the sheet processing apparatus causes the sheet P to switch back, a rear edge corner portion Pg of the sheet P may interfere with the three-hole type die hole portion 2218a (2218e) at the sheet stop position. More specifically, the sheet P causing a switchback motion may be hooked by the three-hole type die hole portion 2218a (2218e). In this case, the switchback operation of the sheet P cannot be performed smoothly and the position accuracy in the formation of the punch holes deteriorates significantly.